1. Field of the Invention
The present invention relates to a light-emitting diode using a flip-chip-type light-emitting element having a widened light-emitting surface.
2. Description of the Related Art
A conventional light-emitting diode 5 which uses a flip-chip-type semiconductor light-emitting element will be described with reference to FIGS. 12 and 13. FIG. 13 is a vertical cross section schematically showing the appearance and structure of the conventional light-emitting diode 5, which comprises a flip-chip-type semiconductor light-emitting element 100 (hereinafter referred to as the xe2x80x9cflip chip 100xe2x80x9d). FIG. 12 depicts a light-emitting element member 570 that is formed of a sub-mount 520 serving as a substrate and the flip chip 100 mounted thereon.
A lead frame 50 is composed of a metal post 51 and a metal stem 53, which are used for application of voltage to the light-emitting element member 570. The metal stem 53 has a reflection portion 55 and a flat portion 54 on which the light-emitting element member 570 is placed. A resin mold 40 encloses the light-emitting element member 570. The bottom surface 527 of the light-emitting element member 570 is bonded to the metal stem 53 by use of silver paste or any other suitable material, to thereby be electrically connected thereto. An electrode 521 is formed on the sub-mount 520 to be located in an exposed portion 528 thereof. The electrode 521 is connected to the metal post 51 through wire bonding using gold wire 57.
Light emitted by the flip chip 100 reflects off a positive electrode disposed on a first main face, passes through a sapphire substrate disposed on a second main face, and then radiates to the outside. Therefore, the flip chip 100 is mounted on the sub-mount 520 in a face-down orientation such that the first main face faces downward.
Next, the sub-mount 520 serving as a substrate will be described. FIG. 12A is a plan view of the sub-mount 520 before attachment of the flip chip 100; FIG. 12B is a plan view of the sub-mount 520 after attachment of the flip chip 100; and FIG. 12C is a cross sectional view of the sub-mount 520 after attachment of the flip chip 100.
The sub-mount 520 is formed of, for example, an electrically conductive semiconductor substrate. The upper surface of the sub-mount 520 is covered with an insulation film 524 made of SiO2 except for a portion 523, to which an Au micro-bump 533 is soldered for establishing connection with the positive electrode of the flip chip 100. A negative electrode 521 is formed on the insulation film 524 by means of aluminum vapor deposition. On the negative electrode 521 are defined a pad region in which the negative electrode 521 is wire-bonded to the metal post 51 and a region in which an Au micro-bump 531 is soldered to the negative electrode 521 in order to establish connection with the negative electrode of the flip chip 100.
Conventionally, in order to perform wire bonding, there must be formed a circular bonding pad region having a diameter of at least 100 xcexcm, or a square bonding pad region, each side of which has a length of at least 100 xcexcm. In order to allow formation of the electrode 521 providing such a bonding pad region on the exposed portion 528 of the sub-mount 520, as shown in FIG. 12B, the flip chip 100 having a square shape must be disposed on the sub-mount 520 at a position offset toward one side. That is, since the exposed portion 528 must be formed to have a predetermined area or greater, the flip chip 100 cannot be disposed on the sub-mount 520 such that the center P2 of the flip chip 100 coincides with the center P501 of the sub-mount 520 and the center axis (indicated by broken line Bxe2x80x94B in FIG. 12B) of the flip chip 100 coincides with the center axis (indicated by broken line Axe2x80x94A in FIGS. 12A and 12A) of the sub-mount 520. Further, when the light-emitting element member 570 is placed on the flat portion 54 having substantially the same area as the light-emitting element member 570, the center axis Axe2x80x94A of the sub-mount 520 inevitably coincides with the center axis (indicated by broken line Dxe2x80x94D in FIG. 13) of the reflection portion 55 having a parabolic shape.
As described above, the sub-mount 520 must have the exposed portion 528 in order to enable formation of the electrode 521 serving as a bonding pad which is used for wiring between the flip chip 100 and the metal post 51.
Therefore, the sub-mount 520 has a rectangular shape. In addition, the flip chip 100 is disposed on the sub-mount 520 in an offset manner, so that the center axis of the flip chip 100 deviates from the center axis of the reflection portion 55 of the lead frame 50. Therefore, the conventional light-emitting diode 5 has a drawback in that luminous intensity changes with position of view; i.e., luminous intensity differs according to whether the diode 5 is viewed from the right side or left side, or from the upper side or lower side.
Further, since the area of the flat portion 54 of the lead frame 50 is small, the area of the sub-mount 520 inevitably becomes small. Therefore, if there is employed a design in which the flip chip 100 is disposed on the sub-mount 520 such that the center axis of the flip chip 100 coincides with that of the rectangular sub-mount 520, and the exposed portion for formation of an attachment electrode is secured, the size of the flip chip 100 decreases, so that a required luminance cannot be obtained.
In view of the foregoing, an object of the present invention is to provide a light-emitting diode which provides constant luminous intensity regardless of position of view.
Another object of the present invention is to provide a light-emitting diode in which the area of a flip chip is maximized in order to secure high luminance, while a region for an electrode for electrical connection is secured on a sub-mount.
Still another object of the present invention is to provide a light-emitting diode which has a reduced overall size and improved durability and which can be fabricated through a simplified fabrication process.
In order to achieve the above-described objects, according to a first aspect of the present invention, there is provided a light-emitting diode using a flip chip which is a flip-chip-type semiconductor light-emitting element, comprising: a rectangular flip chip; and a rectangular sub-mount on which the flip chip is placed. The sub-mount has a shorter side longer than a diagonal of the flip chip. The flip chip is placed on the sub-mount such that a side of the flip chip intersects a corresponding side of the sub-mount.
According to a second aspect of the present invention, there is provided a light-emitting diode using a flip chip which is a flip-chip-type semiconductor light-emitting element, comprising a substantially square flip chip; and a substantially square sub-mount on which the flip chip is placed. The flip chip is placed on the sub-mount at a position and posture which are obtained through superposition of a center point and center axis of the flip-chip on a center point and center axis of the sub-mount and subsequent rotation of the flip chip about the center points by a predetermined angle. Here the term of a substantially square means the figure including a parallelogram, a trapezoid, or a quadrangle which is slightly shifted from a right square.
According to a third aspect of the present invention, the predetermined angle is about 45 degrees.
According to a fourth aspect of the present invention, the sub-mount is formed of a semiconductor substrate, and a diode for over-voltage protection is formed within the semiconductor substrate.
According to a fifth aspect of the present invention, the diode for over-voltage protection is formed to be located below an upper exposed region of the sub-mount.
According to a sixth aspect of the present invention, the sub-mount is formed of a semiconductor substrate having an insulation film formed on an upper surface of the substrate; and at least one of two lead electrodes for the flip chip is formed on the insulation film to be located in an upper exposed region remaining after placement of the flip chip.
According to a seventh aspect of the present invention, a bottom surface of the semiconductor substrate serves as one of two lead electrodes for the flip chip; and the semiconductor substrate is directly connected to a lead frame adapted for receiving the semiconductor substrate and application of voltage to the flip chip.
According to an eighth aspect of the present invention, the semiconductor substrate is insulative; and two lead electrodes for the flip chip are formed on the sub-mount to be located in an upper exposed region remaining after placement of the flip chip.
According to a ninth aspect of the present invention, the sub-mount is insulative; and two lead electrodes for the flip chip are formed on the sub-mount to be located in an upper exposed region remaining after placement of the flip chip.
According to a tenth aspect of the present invention, a mark for detecting position or posture of the sub-mount is formed on the upper exposed region of the sub-mount.
According to an eleventh aspect of the present invention, a reflection film for reflecting light emitted from the flip chip is formed on the sub-mount.
According to a twelfth aspect of the present invention, a lead electrode which is provided for the flip chip and serves as a refection film for reflecting light emitted from the flip chip is formed on the sub-mount.
According to a thirteenth aspect of the present invention, the two lead electrodes are formed to cover an area below the flip chip and serve as refection films for reflecting light emitted from the flip chip.
According to a fourteenth aspect of the present invention, the two lead electrodes are formed to cover substantially the entirety of an upper surface of the sub-mount and serve as reflection films for reflecting light emitted from the flip chip.
In the light-emitting diode according to the first aspect, since the flip chip is disposed on the sub-mount while being rotated with respect thereto, exposed regions not covered by the flip chip are present at four corners of the sub-mount. Electrodes for wiring can be formed in the exposed regions. Accordingly, the optical axis of the flip chip can be placed at an approximate center of the sub-mount, while the area of the flip chip is maximized. As a result, when the sub-mount is placed on a lead frame, the optical axis of the flip chip coincides with an approximate center of the lens frame. In other words, the optical axis of the flip chip coincides with the center axis of a lamp, so that uniform luminous intensity distribution is obtained without sacrifice of luminance.
In the light-emitting diode according to the second aspect, the substantially square flip chip is placed on the substantially square sub-mount at a position and posture which are obtained through superposition of a center point and center axis of the flip-chip on a center point and center axis of the sub-mount and subsequent rotation of the flip chip about the center points by a predetermined angle. Therefore, even when the substantially square flip chip is placed on the substantially square sub-mount such that their centers coincide with each other, triangular exposed regions are formed on the sub-mount, in which lead electrodes can be formed. As a result, without necessity of decreasing the size of the flip chip, the flip chip can be placed on the sub-mount such that their centers coincide with each other, and upper exposed regions used for formation of lead electrodes can be secured on the sub-mount.
Further, since the sub-mount is formed in a substantially square shape, the sub-mount carrying the flip chip can be placed on a lead frame such that the center and center axis of the sub-mount coincide with the center and center axis of a parabola of the lead frame. As a result, constant luminous intensity can be provided regardless of position of view. Since the ratio of the area of the sub-mount to that of the parabola can be maximized, the size of the flip chip itself can be increased. Therefore, without an increase in the size of the light-emitting diode itself, the luminance can be increased.
In the light-emitting diode according to the third aspect, since the angle of rotation is set to about 45 degrees, the ratio of the area of the flip chip to that of the sub-mount can be maximized, and the light-emitting diode can provide further increased luminance.
In the light-emitting diode according to the fourth aspect, the sub-mount is formed of a semiconductor substrate, and a diode for over-voltage protection is formed within the semiconductor substrate. Therefore, the diode for over-voltage protection such as a Zener diode is connected in parallel to the light-emitting diode, and breakage of the light-emitting diode due to excessive voltage is prevented, so that the durability of the light-emitting diode is expectedly improved.
In the light-emitting diode according to the fifth aspect, since the diode for over-voltage protection is formed within the semiconductor substrate to be located below an upper exposed region of the sub-mount, heat is easily radiated from the protection diode, so that thermal breakage of the protection diode is prevented. Since the protection diode is formed outside a region where bumps are formed to establish connection between the flip chip and the sub-mount, the protection diode is not affected by heat generation of bumps, and thermal breakage of the protection diode is effectively prevented.
In the light-emitting diode according to the sixth aspect, electrodes can be formed on the insulation film, and a semiconductor element, such as a diode, for over-voltage protection can be formed within the semiconductor substrate.
In the light-emitting diode according to the seventh aspect, a bottom surface of the semiconductor substrate constituting the sub-mount serves as one of two lead electrodes for the flip chip; and the semiconductor substrate is directly connected to a lead frame adapted for receiving the semiconductor substrate and application of voltage to the flip chip. This structure eliminates necessity of formation of one lead electrode for the flip chip on the sub-mount.
In the light-emitting diode according to the eighth aspect, the semiconductor substrate constituting the sub-mount is insulative; and two lead electrodes for the flip chip are formed on the sub-mount to be located in an upper exposed region of the sub-mount remaining after placement of the flip chip. Since the semiconductor substrate used for the sub-mount may be insulative, the range of selection of constituent materials is widened.
In the light-emitting diode according to the ninth aspect, the sub-mount is insulative; and two lead electrodes for the flip chip are formed on the sub-mount to be located in an upper exposed region remaining after placement of the flip chip. Therefore, the lead electrodes can be wire-bonded to the lead frame used for application of voltage to the flip chip.
In the light-emitting diode according to the tenth aspect, a mark for detecting position or posture of the sub-mount is formed on the upper exposed region of the sub-mount. Therefore, alignment between the flip chip and the sub-mount, and control of position and orientation of the sub-mount during operation for connecting the sub-mount and the lead frame by wire bonding are facilitated.
In the light-emitting diode according to the eleventh aspect, the reflection film reflects light emitted from the flip chip, so that the light can be effectively radiated to the outside.
In the light-emitting diodes according to the twelfth, thirteenth, and fourteenth aspects, since the lead electrodes for the flip chip are used to reflect light emitted from the flip chip, the structure can be simplified, and the light can be effectively radiated to the outside.